Paludicola 10(2):119-135 May 2015 © by the Rochester Institute of Vertebrate Paleontology 119 A LARGE FOREFIN OF ICHTHYOSAURUS FROM THE U.K., AND ESTIMATES OF THE SIZE RANGE OF THE GENUS Judy A. Massare 1 , Dean R. Lomax 2,3 , and Amanda Klein 1 1 Earth Sciences Department, SUNY College at Brockport, Brockport, NY, USA; jmassare@brockport.edu 2 School of Earth, Atmospheric, and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK; dean.lomax@manchester.ac.uk 3 Doncaster Museum and Art Gallery, Chequer Road, Doncaster, DN1 2AE, UK ABSTRACT A large partial forefin (YORYM 2005.2411) from the Lower Jurassic of Yorkshire is assigned to Ichthyosaurus on the basis of the humerus shape, two digits originating from the intermedium, and an anterior digital bifurcation. The humerus is 11.7 cm long and the forefin is 38.5 cm long, but incomplete, probably missing more than 1/3 of its length. Regression analyses suggest that the individual had a jaw length of 56 cm and a total length to the tail bend of almost 3 m. This individual represents the largest Ichthyosaurus reported from the U.K. Although interest in the reptiles of the Yorkshire coast dates back to the early 1800s, specimens of Ichthyosaurus from the area are rare. INTRODUCTION The Lower Jurassic parvipelvian ichthyosaur genus Ichthyosaurus is common in museums throughout the U.K., especially in historical collections. Many specimens have been recovered from west Dorset and Somerset, but the genus has also been identified from Barrow-upon-Soar, Nottinghamshire, Warwickshire, and Yorkshire (Martin et al., 1986; Maisch, 1997; Smith and Radley, 2007; pers. obs. DRL). Large, fragmentary specimens from elsewhere in Europe have been assigned to Ichthyosaurus (Godefroit 1996; Maisch et al., 2008), but see discussion below. Ichthyosaurus is the only Lower Jurassic genus that has a wide forefin with an anterior digital bifurcation (Motani, 1999a), so even isolated forefins are easily distinguished from other genera of the same age. This paper reports on an unusually large partial forefin (YORYM 2005.2411) of a Lower Jurassic ichthyosaur in the collection of the Yorkshire Museum, York, U.K. The specimen was set into a plaster-like filler and surrounded by a wooden frame, with both the plaster and frame painted. This style of mount was common during the 19 th century (pers. obs. DRL, JAM), and so the specimen may have been collected then, but the date of acquisition is not recorded. The specimen has recently been conserved and removed from the original wooden frame (pers. comm. N. Larkin, 2015 DRL). YORYM 2005.2411 probably represents the largest Ichthyosaurus individual known from the U.K., although the rest of the skeleton is not preserved. The humerus and individual forefin elements are larger than in any other Ichthyosaurus forefin that we have observed. Institutional Abbreviations—AGC Alfred Gillett Collection, cared for by the Alfred Gillett Trust (C & J Clark Ltd), Street, Somerset UK; ANSP Academy of Natural Sciences, Drexel University, Philadelphia, USA; BGS British Geological Survey, Keyworth, Nottingham, UK; BRSMG Bristol Museum and Art Gallery, Bristol, UK; BRSUG Bristol University Geology Department, Bristol, UK; CAMSM Sedgwick Museum, Cambridge University, Cambridge, UK; DONMG Doncaster Museum and Art Gallery, Doncaster, South Yorkshire, UK; LEICT Leicester Arts and Museums Service, New Walk Museum and Art Gallery, New Walk, Leicester, UK; LYMPH Philpot Museum, Lyme Regis, UK; MAN Manchester Museum, The University of Manchester, Manchester, UK; MOS Museum of Somerset, Taunton, Somerset, UK; NHMUK (formerly BMNH) Natural History Museum, London, UK; NMS National Museum of Scotland, Edinburgh, UK; NMW National Museum of Wales, Cardiff, UK; OUMNH Oxford University Museum of Natural History, Oxford, UK; ROM Royal Ontario Museum, Toronto, CA; and YORYM Yorkshire Museum, York, UK. GEOLOGICAL SETTING YORYM 2005.2411 was collected from the Lias of Yorkshire, with no further locality information recorded. ‘Lias’ is an old, informal stratigraphic term that referred to the entire Early Jurassic, and was commonly subdivided into the Lower Lias and Upper